In optical communication and optical measurement, a light emitted from a semiconductor laser can reflect at the surface of a member provided on a transmission path and re-enter the semiconductor laser, which makes the laser oscillation instable. To block this reflective optical feedback, an optical isolator using a Faraday rotator, which can non-reciprocally rotate a plane of polarization, is used.
In high-speed communication using the semiconductor laser, an optical isolator usable for a non-polarized light is becoming necessary with integration of multiple components in a module.
A basic structure of a conventional polarization independent optical isolator includes, as shown in FIG. 4, a Faraday rotator 2 and a half-wave plate 4 arranged between two birefringent crystals 6.
FIG. 4(a) shows light traveling in the forward direction. A light incident on one of the birefringent crystals 6 is first separated into an ordinary ray and an extraordinary ray having planes of polarization different by 90 degrees. Then, the polarizations are rotated by the Faraday rotator 2 by 45 degrees and by the half-wave plate 4 by 45 degrees, resulting in a total rotation of 90 degrees so that the ordinary ray and the extraordinary ray are interchanged. Thus, both rays are coupled at the second birefringent crystal 6 (the birefringent crystal 6 on the right in the figure) and exit the optical isolator.
FIG. 4(b) shows light traveling in the backward direction. Like the forward direction, a light incident on one of the birefringent crystals 6 is first separated into an ordinary ray and an extraordinary ray having planes of polarization different by 90 degrees. Then, the polarizations are rotated by the half-wave plate 4 by 45 degrees, and reversely rotated by the Faraday rotator 2 by 45 degrees to the rotational direction of the half-wave plate 4. Thus, the polarization directions of both rays are not changed (from the polarization directions when exiting the birefringent crystal 6), and the rays are not coupled at the second birefringent crystal 6 (the birefringent crystal 6 on the left in the figure), diverging more and more.
The reason is that the half-wave plate rotates a plane of polarization relative to a propagation direction, while the Faraday rotator rotates a plane of polarization relative to a magnetization direction.